When we start to talk about GNU/Linux operating systems' authentication and authorization mechanisms, we need to understand more about two important subsystems of this environment: PAM and NSS.

Pluggable Authentication Modules (PAM) is a modern system that relies on specific libraries to provide us with authentication services. As a general Application Programming Interface (API), all applications that are PAM-aware (for example, builds supporting PAM) can be integrated by the system administrator and can be authenticated using any authentication method that the PAM is configured to. This gives a lot of flexibility to the system administrators and also provides a centralized point to handle the users' authentication very effectively.

In an example scenario where an application is built to use PAM for authentication and the PAM subsystem is configured to authenticate users accessing local files (for example, /etc...